A novel route to rhodaboratranes [Rh(CO)(PR3){B(taz)3}]+ via the redox activation of scorpionate complexes [RhLL′Tt]

The reaction of a mixture of the sodium salts of dihydrobis(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate, NaBt, and hydrotris(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate, NaTt, with [{Rh(cod)(mu-Cl)}(2)] gave [Rh(cod)Bt] and [Rh(cod)Tt], which separately react with CO gas to give the unstable dicarbonyl [Rh(CO)(2)Bt] and an equilibrium mixture of two isomers of [Rh(CO)(2)Tt] and [(RhTt)(2)(mu-CO)(3)], respectively. Tertiary phosphorus donor ligands react with the mixture of [Rh(CO)(2)Tt] and [(RhTt)(2)(mu-CO)(3)] to give [Rh(CO)(PR3)Tt] (R = Cy, NMe2, Ph or OPh) and [Rh{P(OPh)(3)}(2)Tt] in which rhodium is bound to two sulfur atoms of the scorpionate ligand; the B-H bond is directed towards the metal to give an agostic-like B-H center dot center dot center dot Rh interaction. Dinuclear [(RhTt)(2)(mu-CO)(3)] has kappa(3)[S-3]-bound Tt ligands with a rhodium-rhodium bond bridged by three carbonyls. In solution the mononuclear Tt complexes undergo rapid dynamic interchange of the three thioxotriazolyl rings, probably via kappa(3)[S-3]-coordinated intermediates. The monocarbonyls [Rh(CO)(PR3)Tt] (R = Cy, NMe2 or Ph) react with two equivalents of [Fe(eta-C5H5)(2)][PF6] in the presence of triethylamine to give the monocationic rhodaboratranes [Rh(CO)(PR3){B(taz)(3)}](+), with boron NMR spectroscopy providing evidence for the boron-rhodium bond. In the solid state, rhodium is bound to the three sulfur atoms and the boron of the B(taz)(3) fragment, forming a tricyclo[3.3.3.0] cage. The phosphine is trans to the Rh-B bond, the long Rh-P bond indicating a pronounced trans influence for the coordinated boron. The cation [Rh(CO)(PPh3){B(taz)(3)}](+) reacts with [NBu4n]I to give [Rh(PPh3)I{B(taz)(3)}], in which the halide is trans to the Rh-B bond, and a second species, possibly [Rh(CO)I{B(taz)(3)}]. The dirhodaboratrane [Rh-2(PCy3){B(taz)(3)}(2)][PF6](2), a minor byproduct in the synthesis of [Rh(CO)(PCy3){B(taz)(3)}][PF6], has a distorted square pyramidal rhodium atom with a vacant site trans to the Rh-B bond. The second metal has four coordination sites filled by the sulfur and boron atoms of a second B(taz)(3) unit, the remaining octahedral sites occupied by two of the sulfur atoms of the first B(taz)(3) unit which therefore bridges the two rhodium atoms.